Wave Arts Tube Saturator 2 User Manual - page 18
Wave Arts Tube Saturator 2
18
amount of distortion is created due to the curvature of the graph. We also note that
this amplifier is "inverting", that is the output voltage rises when the input voltage
drops and vice-versa.
For large negative or positive grid voltages, the graph flattens out because the
amplifier can only produce a limited range of voltages. This means that large input
signals will result in clipped, or distorted, output signals. Clipping is also called
"saturation", which explains the name "Tube Saturator". The nature of the clipping
in a triode differs for negative and positive inputs.
For large negative grid voltages, the current flow nears "cutoff" where there is no
current flow from the plate, and the output voltage approaches the power supply
voltage, in this case 300V. This type of clipping is called "plate cutoff".
For large positive grid voltages the output voltage reaches a minimum which is
determined by the maximum current the plate can pass. However, this type of
clipping does not occur in practice because another phenomenon usually dominates
the behavior. When the grid voltage is positive with respect to the cathode,
electrons leaving the cathode are attracted to the positively charged grid rather
than repelled by it. The onset of grid current for positive grid voltages is rapid, and
the effect is to immediately decrease the input signal voltage due to current flowing
through the internal resistance of the circuit driving the grid. Hence, grid current
causes the input signal to be clipped. The mechanism of grid current clipping the
input signal determines how the amplifier will clip positive input signals.
Figure 4-6. Complete triode amplifier including cathode bias circuit and DC removal circuit.
A complete triode amplifier schematic is shown in figure 4-6. It has a few additional
details missing from the previous example. First there is a "cathode bias circuit"
whose purpose is to apply a small positive voltage to the cathode, in this case
+1.5V. This allows the grid voltage to assume positive values up to +1.5V without
causing a positive grid to cathode voltage which would cause grid current and input
clipping. Hence for small input signals oscillating say between -1 and +1 volts the